This research project will examine unsteady flow separation in view of its possible role in the genesis, localization, and/or aggrevation of certain forms of arterial disease. Laser Doppler anemometer techniques will be employed to examine criteria for the onset and persistence of unsteady flow separation in the region of a model bifurcation of the ilio-aortic junction in man. Criteria shall be expressed as functions of geometric and flow variables governing the general flow patterns. An examination and quantification of the structure and detailed behavior of blood will be made upstream of, downstream of, and within the separated region of flow. Specific attention will be devoted to evaluating potential consequences of flow separation, such as shear stress distribution of the wall, the appearance of turbulence, mass transport properties of the ensuing flow, Reynolds stresses, etc. Measurements of these fluid properties will permit a basis for the quantitative evaluation of the extent to which potential damage to the arterial wall may be initiated and/or aggravated by the presence of an unsteady wake region in branching tubes having different area ratios.